Electroless plating has been employed for wide variety of substrates for many applications, including electronic device fabrication. The surfaces of such electronic devices often require the formation of a conductor pattern by metal plating. Recently, the Laser Direct Structuring Process (LDS) has been developed and used for the selective plating of molded plastic materials, so called Molded Interconnect Devices (MID). With LDS, it is possible to realize highly functional circuit layouts on complex 3-dimensional substrates. The basis of the process involves additive doped thermoplastics or thermosets with inorganic fillers, which allow the formation of circuit traces by means of laser activation, followed by metallization using electroless plating. The metal containing additives incorporated in such plastics are activated by the laser beam and become active as a catalyst for electroless copper plating on the treated areas of the surface of plastics to be plated. In addition to activation, the laser treatment may create a microscopically rough surface to which the copper becomes firmly anchored during metallization.
However, based on the investigations of the inventors, such substrates are not always easily metalized by a deposition process in which the parts are directly introduced into an electroless copper bath after laser treatment. To ensure that a deposit with the required copper thickness is formed on all areas which have been laser irradiated, a highly reactive electroless copper bath (so-called strike bath) is often needed to form a thin and uniform initial layer, and then the thickness of the copper layer is increased to the required value in another, more stable electroless copper bath (full build bath). Since the strike bath is often operated at conditions that lead to higher consumption of ingredients of the bath and at higher temperature than normal electroless copper baths, the bath life is shorter, leading to the inconvenience of frequently needing to prepare new strike baths.
U.S. Pat. No. 4,659,587 to Imura et al. discloses a selective electroless plating process on the surface of workpieces subjected to a laser beam treatment. The patent discloses that when laser irradiation disrupts the substrate, selective formation of a plated film on the substrate can be effected by immersing it directly in a chemical plating bath, without the need for preliminary activation treatment.
U.S. Pat. No. 7,060,421 to Naundorf et al. discloses a method for producing conductor track structures on a non-conductive material comprising spinel-based metal oxides. The molded non-conductive material disclosed in the document is irradiated by electromagnetic radiation such as from a Nd:YAG laser to break down and release metal nuclei that form patterns that can be plated. After treatment, the irradiated material was washed with water in an ultrasound cleaning bath, after which copper plating was conducted.
U.S. Pat. No. 7,578,888 to Schildmann discloses a method for treating laser-structured plastic surfaces. The patent discloses the laser structured substrates are contacted with a process solution that is suitable for removal of the unintentional deposited metal seeds, prior to introduction into an electroless plating bath, so as to reduce spurious plating in areas of the surface that were not treated with the laser.
However, when the inventors tried the methods disclosed in these US patents and conducted plating with conventional electroless copper plating baths on surfaces which had been laser irradiated, copper deposition on the circuit trace areas was not complete (skip plating). When the inventors used a conventional colloidal catalyst solution before electroless plating, copper was deposited not only on areas which had been laser irradiated but also in non-irradiated areas, so selective plating was not achieved (over plating). Therefore, there is a need for a process of improving the selective electroless metallization of MID-LDS substrates.